Obama: U.S. Involvement in Libya Action Would Last 'Days, Not Weeks'

President Obama told a bipartisan group of members of Congress today that he expects the U.S. would be actively involved in any military action against Libya for "days, not weeks," after which he said the U.S. would take more of a supporting role, sources tell ABC News.

The White House meeting with 18 lawmakers came as Obama delivered an ultimatum to Libyan leader Moammar Gadhafi that he must immediately implement a ceasefire in all parts of Libya and allow international humanitarian assistance, or risk military action against his regime.

"Moammar Gadhafi has a choice. The [U.N.] resolution that passed lays out very clear conditions that must be met. The United States, the United Kingdom, France and Arab states agree that a ceasefire must be implemented immediately. That means all attacks against civilians must stop," the president said today. "Humanitarian assistance must be allowed to reach the people of Libya."

"These terms are not subject to negotiation," Obama said. "If Gadhafi does not comply with the resolution, the international community will impose consequences and the resolution will be enforced through military action."

Sources told ABC News that Obama's decision to support the use of force came Tuesday, following several days of internal administration deliberations and the realization that diplomatic efforts to stop the brutality of Gadhafi's regime weren't working.

Presented with intelligence about the push of the Gadhafi regime to the rebel stronghold of Benghazi, the president told his national security team, "What we're doing isn't stopping him."

Some in his administration, such as Secretary of State Hillary Clinton, had been pushing for stronger action, but it wasn't until Tuesday, administration sources tell ABC News, that the president became convinced sanctions and the threat of a no-fly zone wouldn't be enough.

Obama's speech Friday indicated that coalition forces are giving Gadhafi time to change course, but are also gearing up for an attack if their demands are not met.

He also reiterated that the potential conflict was international in form, saying that any action in Libya would be led by European and Arab forces, and that no ground troops will be deployed.

"We are not going to use force to go beyond a well-defined goal, specifically the protection of civilians in Libya," he said.

The president is mindful that the American public is weary of war, and that the world community casts a skeptical eye at American plans to take military action against yet another Muslim country. Obama has tried hard not to feed into Gadhafi's megalomaniacal worldview by making this confrontation about him versus Obama, or the United States versus Gadhafi, officials say.

The United States has very much been leading the charge behind the scenes, but the White House has deferred public action to the State Department and the United Nations. The administration has also worked furiously to put a European and Arab face on the opposition to Gadhafi's action.

On Saturday, France will host a high level meeting of representatives from the Arab League and European Union to discuss the implementation of the no-fly zone or targeted strikes inside Libya. Secretary of State Hillary Clinton will also attend.